1. Field of the Invention
The present invention relates to a transfer apparatus, an image forming apparatus and a cleaning control method.
2. Description of Related Art
Image forming apparatuses using an electrophotographic process technology (printer, copier, facsimile or the like) generally form an electrostatic latent image by irradiating (exposing) a charged photoconductor with laser light based on image data. Toner is supplied from a developing device to a photoconductor drum (image carrier) on which the electrostatic latent image is formed, and the electrostatic latent image is thereby visualized to form a toner image. Furthermore, the toner image is directly or indirectly transferred to a sheet, heated, pressurized and fixed by a fixing nip, and a toner image is thereby formed on the sheet.
In such an image forming apparatus, for example, a transfer roller which forms the transfer nip between itself and an intermediate transfer belt (image carrier) is in contact with the intermediate transfer belt. For that reason, a toner zone supplied to the intermediate transfer belt when a cleaning member of the intermediate transfer belt is cleaned or a discharge toner zone when deteriorated toner inside the developing device is discharged is likely to be transferred to the transfer roller which is in contact with the intermediate transfer belt. When the toner is transferred to the transfer roller, back surface stain of a sheet may occur when a printing operation is resumed.
In the case of a transfer roller in a configuration without any cleaning mechanism such as a blade or brush, the transfer roller is cleaned by applying a bias to the transfer roller when paper is not passing therethrough and thereby returning the toner on the transfer roller to the intermediate transfer belt.
For example, according to a technique described in Japanese Patent Application Laid-Open No. 2007-241285, when the transfer roller is cleaned, a positive polarity bias and a negative polarity bias corresponding to one circumference of the transfer roller are alternately applied to the transfer roller. This allows the positive polarity toner and negative polarity toner adhering to the transfer roller to be returned to the intermediate transfer belt.
When toner remains on the transfer roller at the time of cleaning, back surface stain of the sheet may occur during printing, and it is therefore necessary to reliably remove stain of the transfer roller. Therefore, a long cleaning time may be set so that the cleaning operation can be continued until stain of the transfer roller disappears.
However, since the appropriate cleaning time varies depending on a degree of toner adhesion to the transfer roller, a degree of deterioration of the surface of the transfer roller and an operating environment of the image forming apparatus, the cleaning operation may be excessively continued in some cases. Since such an excessive cleaning operation is carried out during cleaning of the intermediate transfer belt after the printing operation is stopped, there is a concern that productivity in image formation may deteriorate.